In a cellular mobile telephone system, the various cells of the system are often collected in so-called registration areas. A mobile station is registered initially in a certain area, the home area, and a calling subscriber, possibly from another registration area, is able to reach the mobile station as a result of the base station (BS) and the associated mobile telephone switching centre (MSC:n) having recorded in its home location register (HLR) the registration area in which the called subscriber (the mobile station) is located.
If the mobile station moves to another registration area (so-called roaming), it is important that the mobile station registers itself in the new area and that this new registration is recorded in the home location register of the mobile station. If the mobile station fails to register its new area, it will be extremely difficult for the land system (mobile telephone exchange and home location register) to page the mobile station when this station is called by a calling subscriber. On the other hand, when the new registration identity is recorded in the home location register, the land system is able to establish the registration area in which the subscriber is located, thereby enabling to page the subscriber and set-up a call connection.
It is thus of great importance that each mobile station within the system registers itself as soon as it crosses the border between two registration areas.
The registration areas must not be made too large, because of the balance that needs to be maintained between paging traffic on the one hand (i.e. the traffic created when the mobile subscribers are paged from the land system) and the number of registrations within the area on the other. Small registration areas will preferably be chosen, when the number of registrations is permitted to be high while paging traffic shall be kept low. Larger registration areas can be permitted when the opposite case applies, i.e. when paging traffic can be permitted to be high and the number of registrations shall be kept low. A balance between these two mutually conflicting requirements is made in the majority of cases in a mobile radio system.
As is known, registration is effected by a base station within the service area in which the mobile station is located transmits a global message which contains the registration identity of its own registration area. In the case of the U.S. digital system according to standard "IS-54B" for instance, this message is transmitted over the FOCC-channel (forward analog control channel) and periodically to all mobile stations within the service area. A mobile station reports that it is new in the area and thus that registration is desired, see FIG. 1. This request is normally transmitted over the RECC-channel (reverse analog control channel). Thus, if a mobile station has not stored the transmitted registration identity, it asks to be registered.
Registration creates a particular problem when a mobile station moves in the vicinity of and across the borders of one or more registration areas. A large number of registrations may be entailed by a mobile station crossing several such borders. It will be understood from the aforegoing that a given control channel capacity is used in a registration. It would be desirable to reduce the extent to which the channels of the system are used as much as possible. As before mentioned, the identification message is transmitted from a base station over the control channel FOCC periodically and cannot therefore be altered. However, the number of accesses requesting registration of mobiles over RECC and responsive acknowledgements over FOCC can be influenced when registrations, and therewith the number of subsequent updates "registration update" from the base station, are only required and effected on seldom occassions. This would result in a reduction in the control channel capacity used.
EP-A1-0,439,628, for instance, teaches an improved registration method for reducing and distributing the number of registrations to a given extent. This method is applied in systems in which a base station belongs to several location areas (=registration areas), i.e. several mutually overlapping location areas. According to this method, a list of those identities of the registration areas in the location area to which the base station belongs is transmitted periodically from the base station. These identities are compared with the identity which is stored at that time in a listening mobile station. If none of the identities received and read by the mobile coincide with the identity that is stored in the mobile station the mobile station registers in the registration area, the identity of which is first read from the transmitted list. this procedure reduces the number of registrations, since the mobile station is located in the centre of the new location area instead of at its outer border when making any new registration, i.e. a space hysteresis is generated which avoids oscillating registration.